1. Field of the Invention
The present invention relates generally to telecommunications systems and methods for allocating frequencies among cell clusters, and specifically to reducing the adjacent interference between cells in a four cell frequency reuse plan.
2. Background and Objects of the Present Invention
Cellular telecommunications is one of the fastest growing and most demanding telecommunications applications ever. Today it represents a large and continuously increasing percentage of all new telephone subscriptions around the world. Cellular networks have evolved into two different networks. The European cellular network uses the Global System for Mobile Communication (GSM) digital mobile cellular radio system. In the United States, cellular networks are still primarily analog, but many North American cellular networks have begun deploying SS7 to support access of remote databases. European cellular networks have always relied on SS7 for their signaling requirements. However, GSM is currently operated in North America in a newly reserved frequency band in the 1900 MHZ range. The revised GSM standard is also known as Personal Communication Services 1900 or PCS 1900. FIG. 1 illustrates the typical components of a GSM/PCS 1900 wireless communications system.
The GSM/PCS 1900 wireless communications system is located within a geographical area serviced by a single provider, hereinafter referred to as the Public Land Mobile Network (PLMN) 10. The basic components of the wireless communications system 10 are a Base Station System (BSS) 25, a Mobile Switching Center (MSC) 14 and connected Visitor Location Register (VLR) 16 and a Mobile Station (MS) 20. At least one BSS 25 is deployed within the PLMN 10. The BSS 25 acts as an interface between the MSC 14 and a plurality of MSs 20. The MS 20 may be a mobile wireless telephone, a pager or other equipment.
The MSC/VLR areas 12 include a plurality of Location Areas (LA) 18, which are defined as that part of a given MSC/VLR area 12 in which a mobile station (MS) 20 may move freely without having to send update location information to the MSC/VLR area 12 that controls the LA 18. Each Location Area 12, in turn, is divided into a number of cells 22.
The BSS 25 includes a Base Transceiver Station (BTS) 24 and a Base Station Controller (BSC) 23. At least one BTS 24 operates as a transceiver for transmitting and receiving data and control messages to and from the MS 20 over the air interface within the cell 22. Also connected to the MSC 14 is a Home Location Register (HLR) 26. The HLR 26 is a database maintaining all subscriber information, e.g., user profiles, current location information, International Mobile Subscriber Identity (IMSI) numbers, and other administrative information. The HLR 26 may be co-located with a given MSC 14, integrated with the MSC 14, or alternatively can service multiple MSCs 14, the latter of which is illustrated in FIG. 1.
The VLR 16 is a database containing information about all of the MSs 20 currently located within the MSC/VLR area 12. If a MS 20 roams into a new MSC/VLR area 12, the VLR 16 connected to that MSC 14 will request data about that Mobile Station (terminal) 20 from the HLR database 26 (simultaneously informing the HLR 26 about the current location of the MS 20). Accordingly, if the user of the MS 20 then wants to make a call, the local VLR 16 will have the requisite identification information without having to reinterrogate the HLR 26. In the aforedescribed manner, the VLR and HLR databases 16 and 26, respectively, contain various subscriber information associated with a given MS 20.
Each service provider has a specified number of frequencies which can be used within the PLMN service area 10 assigned to the service provider. These frequencies are divided up among each of the cells 22. Due to the large number of cells 22 and small number of allowed frequencies, frequency re-use patterns are typically used by service providers to support cellular service for all customers within the PLMN service area 10.
As shown in FIG. 2 of the drawings, frequency re-use patterns are cell-based structures 240 by which the frequency channels within a cellular system 205 are assigned. The most basic unit of any frequency re-use pattern is the cell 200. Each cell 200 in a frequency re-use pattern is assigned a number of frequency channels. A group of cells 200 associated together are referred to as a cluster 240, indicated in FIG. 2 by the bold outlines. A cluster 240 contains all of the frequency channels available to a particular cellular system 205. Groups of clusters 240 are then used to provide cellular coverage over a specific area for a cellular system 205. The association of all frequency channels within a single cluster 240 enables the re-use of the frequency channels throughout the cellular system 205.
The particular cell planning structure illustrated in FIG. 2 is a center-excited, sectorized, four cell cluster 240, in which each cluster 240 consists of four cells 200, each of which are further divided into three sectors 210, 220 and 230. Each cell 200 within each cluster 240 is assigned a particular frequency group, which is divided into three sub-frequency groups for each of the three sectors 210, 220 and 230, respectively.
Each cell 200 contains a single antenna site (base station) 250 located substantially at the center of the cell 200. Each base station 240 has six antennas (not shown), two for each sector 210, 220, and 230. One antenna in each sector 210, 220, and 230 sends messages to mobile terminals within the sector 210, 220 and 230, while the other antenna in each sector 210, 220 and 230 receives messages from mobile terminals within the sector 210, 220 and 230. Both antennas in each sector 210, 220, and 230 point along the same direction, e.g., 30, 150, and 270 degrees from north for each of the three sectors 210, 220 and 230 respectively. The direction of the antennas is hereinafter referred to an antenna-pointing azimuth 215, which is represented by an arrow. The antenna-pointing azimuths 215 for each sector 210, 220 or 230 in a cell 200 are separated by 120 degrees.
The clusters 240 are structured and frequencies within the clusters 240 are assigned to increase the re-use distance and to limit co-channel and adjacent channel interferences. Co-channel interference consists of interference caused between uses of the same frequency within two different cell clusters 240. Adjacent channel interference is caused by interference between adjacent cells 200 and frequency channels within the same cluster 240 or within two different clusters 240. In order to reduce interference within the cellular system 205, both co-channel and adjacent channel interference must be minimized. Competing with these requirements is the need for increased system capacity. In general, the smaller the number of cells 200 used in a cluster 240 within a cellular system 205, the higher the capacity of the system and the lower the co-channel re-use distance. A smaller co-channel re-use distance, of course, increases co-channel interference.
In conventional frequency re-use patterns, such as the four cell plan shown in FIG. 2, it has been found that it is not possible to completely avoid allocating adjacent channels to adjacent cells, especially in the corner areas of the sector. A typical channel assignment for the four cell frequency reuse shown in FIG. 2 is illustrated in Table 1 hereinbelow.
TABLE 1 ______________________________________ Frequency Channel Group a1 b1 c1 d1 a2 b2 c2 d2 a3 b3 c3 d3 ______________________________________ Channel 1 2 3 4 5 6 7 8 9 10 11 12 Number 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 ______________________________________
As can be seen in the frequency assignments for a typical four cell frequency reuse plan given in Table 1 above, there are various cell corners where adjacent cells are allocated with adjacent channels, e.g., a1 and b1, b1 and c1, etc. For example, adjacent channels c3 and d3 between two adjacent cells share a common corner, illustrated by the point labeled 260. Similar corners with adjacent channels can be found throughout the frequency reuse plan of FIG. 2. Therefore, tight handover margins may be required in these regions to overcome the potential adjacent channel interference.
It is, therefore, an object of the invention to enhance channel capacity, improve the co-channel interference and substantially eliminate adjacent channel interference problems in a four cell frequency reuse plan.